OEM Plus audio upgrade options for the BMW E30 (E24, E32, E34 as well!)

For those BMW classic fans that may not be aware, Cantaloupe Radio have released a new series of Bluetooth products for the BMW Alpine CM5905, CM5907 and CM5908. In addition to the BA31 Bluetooth board, the 5908 will have additional options including an LCD replacement called Display+, and quite exciting, a replacement for the 5908's internal amplifier called Amp+.  This is a big deal because of it provides clean, high power as well as line level outputs, sub-out and EQ support! If you have interest in Amp+, I wanted to layout the OEM+ audio upgrade options now including Amp+.


  1. Upgrade to the factory BMW SOUND SYSTEM if you don't have it, or keep standard wiring, add a 5908 with Amp+ and just upgrade your speakers
  2. Consider adding a trunk sub as well
  3. If you choose not to use Amp+ with a 5908, or are keeping a 5907, add 4 or 5 channel amplification to replace the factory trunk amp if you have it.

Start with a baseline OEM audio system

Option 1A (5907 or 5908) - upgrade to "BMW SOUND SYSTEM" OEM premium audio, if your car does not already have it

This involves retrofitting the premium sound harness to your car, as well as the premium speakers which include crossovers and tweeters. This gives you the baseline premium audio configuration, 5907 or 5908 with a Blaupunkt "QuadroBoost" trunk amp, higher quality speakers and tweeters.

Most all convertibles already came with BMW SOUND SYSTEM premium audio in the US and Canada, and probably Europe as well (though I'm not sure about the late model 3i8ic). Even with premium audio, the stock rear speakers on convertibles don't have tweeters. So consider adding coaxial speakers to the rear quarters.

Option 1B (5908) - If you have a 'standard', radio only system and plan to add AMP+ you can leave your wiring stock and just upgrade speakers
If you have the standard audio system and will be running a 5908 and Amp+ you have the option of leaving your car’s stereo wiring unmodified. The one exception - if you have 2 channel standard wiring and your car came with the round pin fader, you will need a BMW 2-to-4 wire adapter to run the 5908. You can also add a trunk sub since you have Amp+ - see OPTION 5. What you won't be able to do though is run aftermarket trunk amps as well (OPTION 6), because the wiring from the trunk to the front speakers is missing. So if your want a 5908 and OPTION 6, go with 1A.

Beyond option 1 - using an aftermarket trunk amp (5907 or 5908)

Nothing new here, but these two cases are the baseline options for adding more power to E30 audio systems.  Amplifier tech has improved dramatically in recent years though, and you're still using speaker level inputs from a 30+ year old radio, which has much higher "THD" total harmonic distortion at power than a modern amplifier. Folks have also added "line converters" to their speaker level input wiring which helps a bit if you have enough amplification to keep the radio volume down, but there is no way you can rid of the THD generated by the radio's internal amplifiers.

Option 2 (5907 or 5908) aftermarket trunk amp
Start with the stock premium setup in option 1 above. Do a trunk amp bypass and use the speaker level outputs from your radio as inputs to an aftermarket 4 channel trunk amp

Option 3 (5907 or 5908) 5 channel amplification and trunk sub

This scenario upgrades option 2 with 5 channels of amplification and a trunk subwoofer. You are still using speaker level inputs though, or speaker level radio outputs with line converters to get you a line level input to the amp. There are several ways to do this:

  • use a 5 channel trunk amp and a passive sub
  • use a 4 channel amp with a powered sub
  • use a 4 channel amp and mono block sub amp with a passive sub

Beyond option 1 - using AMP+ (5908 only)

Amp+ is a game changer, no other way to describe it. AVAILABLE LATE 2023

  • a fully digital signal path for your BT and AUX input sources to a new high power amp built into the radio itself
  • tuner outputs go through the digital signal path to the amp as well (but the tuners themselves are still analog)
  • ridiculously low THD - think 100x less distortion at power vs the stock trunk amp and radio amp. The result: SUPER CLEAN, HIGH POWER AUDIO.
  • pre-out and sub-out
  • custom EQ controlled directly from the radio itself, when paired with Display+

There are multiple ways to deploy it, listed below. Again, the higher the option number, the better the OEM+ audio capability.

Option 4 (5908 only) Add Amp+ and if you have BMW SOUND SYSTEM​ do a "trunk amp bypass"

  • Standard sound system: Retrofit Amp+ to a BA31 equipped 5908. Enjoy
  • BMW SOUND SYSTEM: Retrofit Amp+ to a BA31 equipped 5908. Pull your OEM trunk amp and perform a trunk amp bypass. Enjoy

Option 5 (5908 only) Add aftermarket trunk amp and sub, use Amp+ SUB-OUT
Start with option 4. Add a line level sub-out from Amp+ to either a mono block amp and passive trunk sub, or a powered sub

Option 6 (5908 only) Amp+ with aftermarket trunk amps and sub
Start with option 4. Add pre-outs to 5 channels of aftermarket amps
- 4 channel aftermarket trunk amp, powered sub
- or 4 channel aftermarket trunk amp, sub amp with passive sub
- or 5 channel trunk amp with passive sub

The difference between options 5 and 6 will be pretty subjective depending on the amplifiers used. I suspect Amp+ high power audio is cleaner than most mainstream aftermarket trunk amps out there.
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